Unsaturated thermosetting polyester polymers blended with vinyl monomers are well-known molding resins that may be cured under heat and/or pressure to cross-link the vinyl monomer with the unsaturated polyester and form a thermoset plastic molded part. Molding resins often include inert fillers, glass fibers, glass flakes, talcs, and the like for the purpose of obtaining improved impact strength, flexural strength, and rigidity in the molded parts.
A rapidly expanding interest in thermoset molding resin compositions has been generated by the automotive market wherein molding compositions are being utilized for preparing automobile bodies and accessories such as, for example, grill and headlamp housings, fender extensions, and hood scoops. Most conventional thermosetting polyester resins, however, characteristically shrink about 8 to 10% by volume and distort during the press molding process and thus are unsatisfactory despite the many favorable characteristics inherent in polyester molding compositions.
Commonly assigned U.S. Pat. No. 3,883,612 discloses an excellent low-shrink thermosetting polyester resin system based primarily upon a certain dicyclopentadiene-modified thermosetting polyester polymer in combination with a thermoplastic copolymer as a thermoplastic additive. This resin composition provides a uniform stabilized resin mixture adapted to cross-link the unsaturated monomer with the unsaturated thermosetting polyester to form a uniform solid thermoset structure exhibiting excellent low-profile characteristics. Thermoplastic additives for low-shrink thermosetting resin mixtures are often produced by suspension or latex polymerization processes wherein ethylenically unsaturated monomers are copolymerized in an aqueous medium to produce water dispersed polymers. For instance, Example II of U.S. Pat. No. 3,883,612 discloses ethylenically unsaturated monomers polymerized in water to produce thermoplastic suspension polymer particles suspended in water. The polymer particles are then separated from the water by centrifuging and any residual water can be removed by drying and/or vacuum stripping in the presence of styrene. The dried thermoplastic polymer particles are then mixed with styrene and thermosetting polyester to produce a stablilzed low-shrink molding resin mixture. Other similar procedures have been suggested wherein the polymerizing medium is an inert organic solvent such as xylene whereupon the inert solvent is subsequently removed. Although the use of water or organic solvent as a polymerizing medium provides good control over the polymerization process, such processes necessitate the removal of the water or organic solvent prior to mixing the thermoplastic polymer particles with the thermosetting polyester and monomer to form the low-shrink thermosetting resin system.
It now has been discovered that thermoplastic polymer additives for use in low-shrink thermosetting molding resin systems can be efficiently produced by copolymerizing ethylenically unsatural monomers within hydroxy terminated low molecular weight polymeric diluent non-reactive with said monomers and subsequently reacting the hydroxyl containing polymeric diluent with an isocyanate to form a polyurethane copolymer. Use of a polymeric diluent provides a suitable polymerizing medium for the vinyl copolymer and further eliminates the process step of removing the water or solvent polymerizing medium. The process is particularly remarkable in that the resulting vinyl copolymer dispersed in the polymeric diluent can be further reacted with isocyanate whereby the resulting polyurethane-vinyl copolymer mixture can be directly utilized in thermosetting polyester resins to produce an excellent low-shrink molding resin system.